This invention relates to a magnetically anisotropic magnet (hereinafter xe2x80x9canisotropic magnetxe2x80x9d) having excellent magnetic characteristics such as a high magnetic flux density, a process for producing the same, and a motor having the same. The term xe2x80x9cmagnetxe2x80x9d is used herein to include a shaped or formed magnet precursor before magnetization.
Having a high energy product, rare earth magnets have been extending their market with expectation of providing motors and the like with improved performance and reduced sizes. Sintered Ndxe2x80x94Fexe2x80x94B magnets currently available on the market have a maximum energy product of about 50 MGOe at the most. With the ever-increasing demands for motors, etc. having higher performance and yet affording energy savings, the magnet used therefor has also been required to have higher performance, especially a higher magnetic flux density.
The maximum energy product of a magnet can be obtained by calculation from the saturation magnetization of the material making the magnet. For example, the maximum energy product that could be reached theoretically by a Ndxe2x80x94Fexe2x80x94B magnet as calculated based on the saturation magnetization of tetragonal Nd2Fe14B, the predominant phase thereof (=16.0 kG), is 64.0 MGOe (=(16.0/2)2). The maximum energy product of available Ndxe2x80x94Fexe2x80x94B magnets in the practice has been improved toward this theoretical value and raised to about 50 MGOe as noted above, which is seen as approaching to a substantial limit that could be reached. Hence, in order to achieve great improvements on magnetic characteristics, other approaches different from conventional manipulations have been sought.
Materials for Ndxe2x80x94Fexe2x80x94B magnets which may achieve a maximum energy product exceeding the above practical limit are reported in Physical Review B, vol. 48, No. 21, pp. 15812-15816 (1993). The magnet reported is called an exchange spring magnet, in which Ndxe2x80x94Fexe2x80x94B crystals and a soft magnetic phase having a high saturation magnetization, such as an xcex1-Fe phase, are finely dispersed to enhance the interaction between the two phases thereby achieving both a high coercive force of the magnet and an increased magnetic flux density of the xcex1-Fe phase.
If the Ndxe2x80x94Fexe2x80x94B crystals have a large grain size, the above-described effect cannot be produced however. In order to draw forth the interaction between the two phases, it is generally required to use fine crystal grains as small as 1 xcexcm or less. Preparation of fine particles by a rapid solidification process or preparation of a thin film of fine crystals by sputtering has been attempted to make an exchange spring magnet. There is a report that these approaches have succeeded in producing the interaction between the two phases, but, in either case, the crystals forming the resulting magnet are randomly oriented. That is, the resulting magnets are magnetically isotropic, having the N- and S-poles directions at random, and therefore fail to exhibit high magnetic characteristics.
The following approach is also conceivable for obtaining an anisotropic exchange spring magnet. In the production of a Ndxe2x80x94Fexe2x80x94B magnet, a Ndxe2x80x94Fexe2x80x94B alloy powder having a particle size of several microns is pressed in a magnetic field before sintering to prepare a magnetically orientated green body. In this process, if the Ndxe2x80x94Fexe2x80x94B powder mixed with Fe powder is pressed in a magnetic field followed by sintering, there seems to be a possibility of making an anisotropic exchange spring magnet in which the Ndxe2x80x94Fexe2x80x94B particles and the Fe particles exert mutual actions.
In the practice, however, this method fails to produce an anisotropic exchange spring magnet because the Ndxe2x80x94Fexe2x80x94B particles before sintering are too large as having several microns to produce the interaction.
Journal of Magnetism and Magnetic Materials, vol. 84, pp. 88-94 (1990) proposes subjecting a magnetic material to hot plastic forming to obtain an anisotropic magnet. According to the report, rapid solidification-processed Ndxe2x80x94Fexe2x80x94B-based powder whose Nd content is higher than the stoichiometric one of Nd2Fe14B is compacted by hot pressing, and the resulting bulk is plastically deformed by upsetting to provide an anisotropic magnet with a raised maximum energy product (BH)max, in which the originally isotropic magnetic material has been rendered anisotropic.
The mechanism of the change from isotropy to anisotropy is that the Nd2Fe14B crystals surrounded by a Nd-rich boundary phase grow with grain boundary sliding in plastic deformation and are thereby orientated to show anisotropy. Having a low melting point around 600xc2x0 C., the Nd-rich boundary phase is melted in hot plastic forming so that it seems to serve like a lubricant and also function as an accelerator for crystal growth. This technique of making a magnetic material anisotropic by hot plastic forming cannot be applied without difficulty to production of the Ndxe2x80x94Fexe2x80x94B exchange spring magnet having a high Fe concentration because of absence of such a Nd-rich boundary phase.
It is known that the temperature in hot plastic forming is greatly influential on the degree of anisotropy obtained (see IEEE Transactions on Magnetics, vol. 35, No. 5, pp. 3268-3270 (1999)). According to the report, the optimum plastic forming temperature in conventional production of anisotropic magnets is about 800xc2x0 C., and the degree of anisotropy achieved is reduced at either lower or higher temperatures only to provide a magnet with a reduced residual magnetization Br. Reduction in degree of anisotropy in lower or higher temperatures than the optimum one may be accounted for as follows. At lower plastic forming temperatures, crystal growth with grain boundary sliding, which is to lead to anisotropy, hardly occurs in plastic forming. On the other hand, higher temperatures allow crystal grains to grow in random directions before plastic forming is effected. As a result, the grain growth by plastic forming would not be so appreciable as could have been. That is, production of an anisotropic magnet by conventional hot plastic forming essentially requires the presence of a Nd-rich boundary phase, and the temperature of the hot plastic forming is about 800xc2x0 C.
An object of the present invention is to provide an anisotropic magnet having excellent magnetic characteristics such as a high magnetic flux density, a process for producing the anisotropic magnet, and a motor having the anisotropic magnet.
The inventors of the present invention have made intensive researches into materials and production conditions for a Ndxe2x80x94Fexe2x80x94B exchange spring magnet which is difficult to make anisotropic by known techniques on account of its high Fe concentration and absence of Nd-rich grain boundaries. As a result, they have found that the above object of the invention is accomplished by using a material having a specific composition.
Thus, the present invention provides:
(1) An anisotropic magnet which comprises a Nd2Fe14B phase and an xcex1-Fe phase and the compositional formula of Nd, Fe and B satisfies NdxFe100-x-yBy wherein x is 2 to 10 atomic percent and y is 1 to 5 atomic percent.
(2) An anisotropic magnet as described in (1) above, wherein x is 5 to 7 atomic percent and y is 1 to 5 atomic percent.
(3) An anisotropic magnet as described in (1) or (2) above, which has 30 atomic percent or less of Fe displaced with Co.
(4) An anisotropic magnet as described in any one of (1) to (3) above, which has 1 atomic percent or less of Fe displaced with at least one element selected from Nb, V, Ti, Cr, Mo, Ta, W, Zr, and Hf.
(5) An anisotropic magnet as described in any one of (1) to (4) above, which has 50 atomic percent or less of Nd displaced with at least one rare earth element selected from Pr, Ce, Dy, and Tb.
(6) An anisotropic magnet as described in any one of (1) to (5) above, which is a molded mixture of powder comprising the Nd2Fe14B phase and the xcex1-Fe phase and a binder resin.
(7) An anisotropic magnet as described in any one of (1) to (6) above, wherein the anisotropic magnet or magnetic powder part thereof has a saturation magnetization of 15.5 kG or higher and an intrinsic coercive force of 4 to 30 kOe.
(8) The anisotropic magnet as described in (1) above, which further comprises a Ndxe2x80x94Cu phase and the compositional formula of Nd, Fe, B and Cu satisfies NdxFe100-x-y-zByCuz wherein x is 2 to 10 atomic percent, y is 1 to 5 atomic percent, and z is 0.5 to 10 atomic percent.
(9) An anisotropic magnet as described in (8) above, wherein x is 2 to 7 atomic percent, y is 1 to 5 atomic percent, and z is 0.5 to 10 atomic percent.
(10) An anisotropic magnet as described in (8) or (9) above, which has 30 atomic percent or less of Fe displaced with Co.
(11) An anisotropic magnet as described in any one of (8) to (10) above, which has 50 atomic percent or less of Cu displaced with at least one element selected from Mg, Al, Si, P, Zn, Ge, Sb, Sn, and Ni.
(12) An anisotropic magnet as described in any one of (8) to (11) above, which has 50 atomic percent or less of Nd displaced with at least one rare earth element selected from Pr, Ce, Dy, and Tb.
(13) An anisotropic magnet as described in any one of (8) to (12) above, which is a molded mixture of powder comprising the Nd2Fe14B phase, the xcex1-Fe phase, and the Ndxe2x80x94Cu phase and a binder resin.
(14) An anisotropic magnet as described in any one of (8) to (13) above, which or magnetic powder of which has a saturation magnetization of 15.5 kG or higher and an intrinsic coercive force of 7 to 35 kOe.
(15) A process of producing an anisotropic magnet, which comprises a step of plastic forming a powder or a bulk of powder which comprises a Nd2Fe14B phase and an xcex1-Fe phase and the compositional formula of Nd, Fe and B satisfies NdxFe100-x-yBy wherein x is 2 to 10 atomic percent and y is 1 to 5 atomic percent, at a temperature ranging from 900xc2x0 to 1100xc2x0 C.
(16) A process of producing an anisotropic magnet including the step of plastic forming powder or a bulk of powder which comprises a Nd2Fe14B phase, an xcex1-Fe phase, and a Ndxe2x80x94Cu phase and the compositional formula of Nd, Fe, B and Cu satisfies NdxFe100-x-y-zByCuz wherein x is 2 to 10 atomic percent, y is 1 to 5 atomic percent, and z is 0.5 to 10 atomic percent, at a temperature ranging from 700xc2x0 to 1100xc2x0 C.
(17) A process of producing an anisotropic magnet as described in (15) or (16) above, wherein said plastic forming is upsetting or extrusion.
(18) A process of producing an anisotropic magnet as described in (15) or (16) above, wherein said plastic forming is preceded by cold pressing and hot pressing.
(19) A process of producing an anisotropic magnet as described in (15) or (16) above, wherein said plastic forming is followed by the step of grinding the resulting anisotropic magnet to powder, mixing the powder with a binder resin, and molding the mixture by injection molding or compression molding in a magnetic field.
(20) A motor which comprises the anisotropic magnet as described in any one of (1) to (14) fitted to a rotor or a stator as a permanent magnet.